Windows Phone 8.1 officially introduced by Microsoft [UPDATED]

Microsoft's Joe Belfiore took to the Build stage today to kick off his company's developer conference, and just as expected, he took the opportunity to formally introduce us to Windows Phone 8.1.

The first feature demonstrated by Belfiore was Action Center. As we've seen in leaked images and video, Action Center houses your app notifications as well as a panel of quick settings toggles.

Another major feature of Windows Phone 8.1 is lock screen themes. This feature, which was made by Microsoft and Windows Phone developer Rudy Huyn, gives users the ability to customize the way that they unlock their device. One example shown by Belfiore today was a lock screen with diagonal date and time info that, when the screen is unlocked, slides away from view.

Once unlocked, users are presented with the Start screen. A new Show More Tiles setting allows users to select the size and number of Live Tiles that are displayed on their Start screen. Windows Phone 8.1 also includes the ability to set a background image on the Start screen, meaning that users can now customize their Live Tiles beyond selecting a different accent color.

Once you've unlocked your phone and set up your Start screen just the way that you like it, it's time to start doing stuff, right? One new feature that can help you to do that is Cortana. Cortana is Windows Phone's personal assistant that can do things like set reminders, launch apps, create calendar events and more. Here's a quick rundown of some of Cortana's features:

Cortana's Notebook stores information related to interests, inner circle, quiet hours and more. The app will learn this information as you use it, but you can also set it manually.

User can allow Cortana to scan emails for information that she might track. For example, Cortana can read a flight itinerary and update you with changes.

Cortana can also set alarms, create and read back calendar events, use Yelp data to find restaurants, launch other apps

Cortana can remind you to speak to a friend or family member about a specific topic when you talk to them

Another new app introduced today by Microsoft is Wi-Fi Sense, which makes it easier for users to sign into Wi-Fi networks. Wi-Fi Sense can automatically log you in to free Wi-Fi networks, even if it's got a special portal or requires information like an email address or name. The app can also be used to share your personal protected Wi-Fi network with friends, giving them access to your Internet connection but preventing them from accessing files on your network.

Microsoft has also updated several of its existing apps with the Windows Phone 8.1 update. The Windows Phone Store is getting a makeover with a new Featured tab that can better highlight new apps, a personalized For You recommendation page and new sections for things like Top Free and Top Paid.

Another existing Windows Phone feature that's gaining a new trick in Windows Phone 8.1 is the keyboard. Microsoft is adding WordFlow typing to its keyboard that allows users to slide their finger around to enter letters, similar to the Swype and SwiftKey Flow keyboards that are found on other platforms.

Microsoft has also tweaked Internet Explorer in Windows Phone 8.1. The updated IE 11 browser will gain several new features with the WP8.1 update, including a private browsing mode as well as a new reading mode that'll make it easier to read dense web pages.

Finally, the arrival of Windows Phone 8.1 will also bring Universal Windows Apps. This feature will allow developers to use tools that can create one app that will run on Windows Phone 8.1 and Windows 8.1 devices.

So when can you try Windows Phone 8.1 and all of its new features for yourself? Microsoft says that the update will begin rolling out to existing Windows Phone 8 devices in the "next few months." You won't have to wait quite as long if you'd rather treat yourself to a new handset to go along with the new software, because Microsoft expects phones preloaded with WP8.1 to start launching in late April or early May.

We may have already known about several Windows Phone 8.1 features before today's event because of leaks, but Microsoft still managed to surprise us with its lock screen themes and some Cortana features. Now that Windows Phone 8.1 is officially official, what are your first impressions of the update? Does it make you excited for Microsoft's mobile platform?

UPDATE: Microsoft says that the Windows Phone 8.1 update will be made available to registered developers this month. A recent rumor suggested that the update could hit early in April.

UPDATE 2: Microsoft and Nokia have spilled a few more details of the Windows Phone 8.1 update. Microsoft has confirmed the following additional WP8.1 features:

All Windows Phone devices will be able to add a third column of Live Tiles with the 8.1 update

Music, Video and Podcasts will all be their own separate apps

People Hub has larger photos

Email supports S/MIME secure email

Support for iCloud accounts

Photos app jumps to most recent shots

Online photo albums supported in Photos app

Camera app features new UI and burst mode

Data Sense app gaining "high savings" mode that will increase image compression while using the web to save more megabytes

Storage Sense can manage content on the device or a microSD card

Battery Saver displays how much power each app consumes

Windows Phone 8.1 update will be available in Developer Preview Program in first part of April

Meanwhile, Nokia has confirmed that it will bundle Windows Phone 8.1 with its own software tweaks into an update called "Lumia Cyan." This follow-up to Lumia Amber and Lumia Black will include all of the WP8.1 features as well as improvements to the camera roll and Creative Studio apps and a new version of the Nokia Storyteller app. Nokia says that it will make Windows Phone 8.1 available to all of its Windows Phone 8-powered Lumia devices starting this summer.

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